An electric power steering apparatus which provides a steering mechanism of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies a driving force of the motor as the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the steering assist torque, such a conventional electric power steering apparatus (EPS) performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of duty command values of a PWM (Pulse Width Modulation) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a steering wheel (a steering handle) 1, is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 100 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 100 through an ignition key 11. The control unit 100 calculates a steering assist command value of an assist command on the basis of a steering torque Th detected by the torque sensor 10 and a vehicle velocity Vel detected by a vehicle velocity sensor 12, and controls a current supplied to the motor 20 on the basis of a current control value E obtained by performing compensation and so on with respect to the steering assist command value. Moreover, it is also possible to receive the vehicle velocity Vel from a CAN (Controller Area Network) and so on.
In such an electric power steering apparatus, for example, the control unit 100 has a configuration disclosed in Japanese Published Unexamined Patent Application No. 2002-369565 A.
As shown in FIG. 2, the motor 20 for generating the steering assist torque of the steering apparatus is driven by a motor driving section 21, the motor driving section 21 is controlled by the control unit 100 indicated by a dashed-two dotted line, and the steering torque Th from the torque sensor 10 and the vehicle velocity Vel from a vehicle velocity detecting system are inputted into the control unit 100. In the motor 20, a voltage between motor terminals Vm and a motor current value i are measured and outputted.
The control unit 100 comprises a torque system control unit 110 indicated by a dashed line that performs a control by using the steering torque Th and a motor system control unit 120 indicated by a dashed-dotted line that performs a control relating to driving of the motor 20. The torque system control unit 110 comprises an assist amount calculating section 111, a differential control section 112, a yaw rate convergence control section 113, a robust stabilization compensating section 114 and an SAT (Self Aligning Torque) estimation feedback section 115, addition sections 116A and 116B, and a subtraction section 116C. Further, the motor system control unit 120 comprises a compensating section 121, a disturbance estimating section 122, a motor angular velocity calculating section 123, a motor angular acceleration calculating section 124, a motor characteristic compensating section 125, and addition sections 126A and 126B.
The steering torque Th is inputted into the assist amount calculating section 111, the differential control section 112, the yaw rate convergence control section 113 and the SAT estimation feedback section 115, and all of them input the vehicle velocity Vel as a parameter. The assist amount calculating section 111 calculates an assist torque amount on the basis of the steering torque Th. The yaw rate convergence control section 113 inputs the steering torque Th and a motor angular velocity co, and brakes a movement that the steering wheel whirls to improve the convergence of yaw of the vehicle. Further, the differential control section 112 enhances a control responsibility in the vicinity of a neutral point of the steering and realizes a smooth steering. Moreover, the SAT estimation feedback section 115 inputs the steering torque Th, a signal obtained in the addition section 116A by adding the output of the differential control section 112 to the output of the assist amount calculating section 111, the motor angular velocity ω calculated by the motor angular velocity calculating section 123 and a motor angular acceleration α from the motor angular acceleration calculating section 124 to estimate an SAT, performs a signal processing by using a feedback filter with respect to the estimated SAT, and provides the steering wheel with a suitable road information as a reaction force.
Further, a signal that is obtained in the addition section 116B by adding the output of the yaw rate convergence control section 113 to a signal obtained in the addition section 116A by adding the output of the differential control section 112 to the output of the assist amount calculating section 111, is inputted into the robust stabilization compensating section 114 as an assist amount AQ. For example, the robust stabilization compensating section 114 is a compensating section disclosed in Japanese Published Unexamined Patent Application No. H8-290778 A, removes a peak value in a resonance frequency of a resonance system comprised of an inertia element and a spring element that are included in the detected torque, and compensates a phase shift of the resonance frequency that harms the responsiveness and the stability of the control system. By subtracting the output of the SAT estimation feedback section 115 from the output of the robust stabilization compensating section 114 in the subtraction section 116C, an assist amount Ia capable of transmitting the road information to the steering wheel as the reaction force, is obtained.
Moreover, the motor angular velocity calculating section 123 calculates the motor angular velocity ω on the basis of the motor inter-terminals voltage Vm and the motor current value i, and the motor angular velocity ω is inputted into the motor angular acceleration calculating section 124, the yaw rate convergence control section 113 and the SAT estimation feedback section 115. The motor angular acceleration calculating section 124 calculates the motor angular acceleration α on the basis of the inputted motor angular velocity co, and the calculated motor angular acceleration α is inputted into the motor characteristic compensating section 125 and the SAT estimation feedback section 115. In the addition sections 126A, the assist amount Ia obtained by subtracting the output of the SAT estimation feedback section 115 from the output of the robust stabilization compensating section 114, is added to the output Ic of the motor characteristic compensating section 125, and then this added signal is inputted into the compensating section 121 comprised of a differential compensating section or the like as a current command value Ir.
A signal that is obtained by adding the output of the disturbance estimating section 122 in the addition section 126B to a current command value Ira obtained by compensating the current command value Ir by means of the compensating section 121, is inputted into the motor driving section 21 and the disturbance estimating section 122. The disturbance estimating section 122 is an apparatus disclosed in Japanese Published Unexamined Patent Application No. H8-310417 A, is capable of maintaining a desired motor control characteristic in an output reference of the control system on the basis of a signal obtained by adding the output of the disturbance estimating section 122 to the current command value Ira compensated by the compensating section 121 that is the control target of the motor output and the motor current value i, and does not lose the stability of the control system.
In such an electric power steering apparatus, recently, vehicles equipped with a parking support function (parking assist) that switches the automatic steering control and the manual steering control appear. In a vehicle equipped with the parking support function, a target steering angle is set on the basis of data from a camera (image), a distance sensor or the like, and the automatic steering control which makes an actual steering angle follow up the target steering angle, is performed.
In an electric power steering apparatus having conventionally well-known functions of the automatic steering control (parking support mode) and the manual steering control, back parking and parallel parking are performed automatically by controlling an actuator (a motor) on the basis of a pre-stored relation between a moving distance of the vehicle and a turning angle.
Then, a conventional steering control apparatus detects the actual steering angle from a steering angle sensor provided on the steering wheel, calculates a motor current command value so as to make the actual steering angle coincide with the target steering angle set dependent on the vehicle, and realizes the automatic steering control. For example, in an automatic steering device disclosed in Japanese Patent No. 4097134 B2 (Patent Document 1), in the case that the magnitude of a tire torsion force obtained by a tire torsion force calculating means is less than or equal to a friction torque of the steering mechanism, a motor's drive current value generated is gradually decreases to zero to control.